Corrugated webs possess increased strength and dimensional stability compared to un-corrugated (i.e. flat) webs of the same material. For example, corrugated paperboard or cardboard is widely used in storage and shipping boxes and other packaging materials to impart strength. A typical corrugated cardboard structure known as ‘double-wall’ includes a corrugated paperboard web sandwiched between opposing un-corrugated paperboard webs referred to as ‘liners.’ The opposing liners are adhered to opposite surfaces of the corrugated web to produce a composite corrugated structure, typically by gluing each liner to the adjacent flute crests of the corrugated web. This structure is manufactured initially in planar composite boards, which can then be cut, folded, glued or otherwise formed into a desired configuration to produce a box or other form for packaging.
Corrugated webs such as paperboard are formed in a corrugating machine starting from flat webs. A conventional corrugating machine feeds the flat web through a nip between a pair of corrugating rollers rotating on axes that are perpendicular to the direction of travel of the web when viewed from above. Each of the corrugating rollers has a plurality of longitudinally-extending teeth defining alternating peaks and valleys distributed about the circumference and extending the length of the roller. The rollers are arranged so that their respective teeth interlock at the nip, with the teeth of one roller being received within the valleys of the adjacent roller. The interlocking teeth define a corrugating labyrinth through which the web travels as it traverses the nip. As the web is drawn through the corrugating labyrinth it is forced to conform to the configuration thereof, thus introducing into the web flutes or corrugations that approximate the dimensions of the pathway through the corrugating labyrinth. An example of this conventional methodology is shown in U.S. Pat. No. 8,057,621 (see FIGS. 7 and 7a thereof), which is incorporated herein by reference in its entirety.
Corrugating a web in this manner can introduce a substantial amount of oscillatory frictional and tension forces to the web leading into and while traversing the corrugating nip. Briefly, as the web is drawn between the corrugating rollers and forced to negotiate the corrugating labyrinth, tensile stresses in the web, as well as compressive stresses normal to the plane of the entering web, oscillate in magnitude and direction as successive flutes are formed due to the reciprocating motion of the corrugating teeth relative to the web, and due to roll and draw variations in the web through the labyrinth as it is being corrugated. The resulting cyclic peaks in web stresses can produce structural damage in the web as it is corrugated. Structural damage is particularly likely if sharp edges are present along the teeth of the corrugating rollers.
Therefore, in order to limit stresses in the web during corrugation, the teeth in conventional corrugating rollers are shaped to have a sinusoidal profile such that no sharp edges, nor discrete edges whose radii of curvature approach or approximate a sharp edge, are present along the teeth. Consequently, the final corrugated web will also have a continuous, smooth sinusoidal shape. However, layered structures made with such sinusoidal-corrugated webs can be inferior in quality to layered structures made with webs having other corrugated shapes.
More specifically, a layered cardboard structure in which a web having trapezoidal-shaped corrugations is sandwiched between flat liners can be vastly superior in strength compared to a similar layered cardboard structure having a web with sinusoidal-shaped corrugations. For example, the straight legs of trapezoidal-shaped corrugations extending between the liners can be more resistant to compression than the curved legs of sinusoidal-shaped corrugations. Furthermore, the flat peaks and valleys of trapezoidal-shaped corrugations can provide a greater surface area for adhesion to the opposing liners than the rounded peaks of sinusoidal-shaped corrugations. This greater surface area can provide enhanced adhesion between the corrugated web and outer layers, thereby creating a more rigid structure that is more resistant to tearing, bending, and falling apart.
As desirable as trapezoidal-shaped corrugations for a web may be, such corrugations are difficult to achieve using conventional techniques for corrugating. For example, feeding a flat web to a pair of corrugating rollers having closely interlocking trapezoidal-shaped teeth would impart too much stress to the web due to the discrete edges of the teeth and the dramatic change in shape to the web, thereby damaging the web.